Non-thermal atmospheric plasma synthesis of ammonia in a DBD reactor packed with various catalysts

Plasma synthesis of ammonia (NH₃) is a potential and sustainable pathway for nitrogen fixation. In this study, non-thermal plasma (NTP) synthesis of NH₃ was conducted in a packed-bed dielectric barrier discharge (DBD) reactor. Different catalysts, including alumina (Al₂O₃), light magnesium oxide (L-MgO), heavy magnesium oxide (H-MgO), Ru/Al₂O₃ and Ru/L-MgO, were filled in the reactor and compared for NH₃ production. Various characterization techniques were used to determine the specific surface area and pore size, surface morphology, and surface acidity of the catalysts. The effects of reaction conditions on NH₃ production were examined. Results showed that the optimal N₂/H₂ volume ratio was 2:1 since a greater quantity of N₂ favored the generation of plasma-excited nitrogen species. The synthesized NH₃ can be adsorbed by the acid sites on Al₂O₃, which reduced the NH₃ yield. Higher total gas flow rate improved the NH₃ production over various catalysts, mainly due to the reduced external diffusion resistance. More plasma-excited nitrogen and hydrogen species for NH₃ synthesis were formed at higher discharge power, yet the increasing rate of NH₃ yield became slower when the discharge power was higher than 32 W. Additionally, the NH₃ desorption from the catalyst was enhanced at higher temperature, which resulted in higher NH₃ yield.